Nonvolatile Modulation of Bi2O2Se/Pb(Zr,Ti)O3 Heteroepitaxy.

The pursuit of high-performance electronic devices has driven the research focus toward 2D semiconductors with high electron mobility and suitable band gaps. Previous studies have demonstrated that quasi-2D Bi2O2Se (BOSe) has remarkable physical properties and is a promising candidate for further exploration. Building upon this foundation, the present work introduces a novel concept for achieving nonvolatile and reversible control of BOSe's electronic properties. The approach involves the epitaxial integration of a ferroelectric PbZr0.2Ti0.8O3 (PZT) layer to modify BOSe's band alignment. Within the BOSe/PZT heteroepitaxy, through two opposite ferroelectric polarization states of the PZT layer, we can tune the Fermi level in the BOSe layer. Consequently, this controlled modulation of the electronic structure provides a pathway to manipulate the electrical properties of the BOSe layer and the corresponding devices.

Enhanced Electrical Transport Properties of Molybdenum Disulfide Field-Effect Transistors by Using Alkali Metal Fluorides as Dielectric Capping Layers.

The electronic properties of 2D materials are highly influenced by the molecular activity at their interfaces. A method was proposed to address this issue by employing passivation techniques using monolayer MoS2 field-effect transistors (FETs) while preserving high performance. Herein, we have used alkali metal fluorides as dielectric capping layers, including lithium fluoride (LiF), sodium fluoride (NaF), and potassium fluoride (KF) dielectric capping layers, to mitigate the environmental impact of oxygen and water exposure. Among them, the LiF dielectric capping layer significantly improved the transistor performance, specifically in terms of enhanced field effect mobility from 74 to 137 cm2/V·s, increased current density from 17 µA/µm to 32.13 µA/µm at a drain voltage of Vd of 1 V, and decreased subthreshold swing to 0.8 V/dec The results have been analytically verified by X-ray photoelectron spectroscopy (XPS) and Raman, and photoluminescence (PL) spectroscopy, and the demonstrated technique can be extended to other transition metal dichalcogenide (TMD)-based FETs, which can become a prospect for cutting-edge electronic applications. These findings highlight certain important trade-offs and provide insight into the significance of interface control and passivation material choice on the electrical stability, performance, and enhancement of the MoS2 FET.

Alkali metal bilayer intercalation in graphene.

Alkali metal (AM) intercalation between graphene layers holds promise for electronic manipulation and energy storage, yet the underlying mechanism remains challenging to fully comprehend despite extensive research. In this study, we employ low-voltage scanning transmission electron microscopy (LV-STEM) to visualize the atomic structure of intercalated AMs (potassium, rubidium, and cesium) in bilayer graphene (BLG). Our findings reveal that the intercalated AMs adopt bilayer structures with hcp stacking, and specifically a C6M2C6 composition. These structures closely resemble the bilayer form of fcc (111) structure observed in AMs under high-pressure conditions. A negative charge transferred from bilayer AMs to graphene layers of approximately 1~1.5×1014 e-/cm-2 was determined by electron energy loss spectroscopy (EELS), Raman, and electrical transport. The bilayer AM is stable in BLG and graphite superficial layers but absent in the graphite interior, primarily dominated by single-layer AM intercalation. This hints at enhancing AM intercalation capacity by thinning the graphite material.

High-Performance Monolithic 3D Integrated Complementary Inverters Based on Monolayer n-MoS2 and p-WSe2.

Herein, the structure of integrated M3D inverters are successfully demonstrated where a chemical vapor deposition (CVD) synthesized monolayer WSe2 p-type nanosheet FET is vertically integrated on top of CVD synthesized monolayer MoS2 n-type film FET arrays (2.5 × 2.5 cm) by semiconductor industry techniques, such as transfer, e-beam evaporation (EBV), and plasma etching processes. A low temperature (below 250 °C) is employed to protect the WSe2 and MoS2 channel materials from thermal decomposition during the whole fabrication process. The MoS2 NMOS and WSe2 PMOS device fabricated show an on/off current ratio exceeding 106 and the integrated M3D inverters indicate an average voltage gain of ≈9 at VDD = 2 V. In addition, the integrated M3D inverter demonstrates an ultra-low power consumption of 0.112 nW at a VDD of 1 V. Statistical analysis of the fabricated inverters devices shows their high reliability, rendering them suitable for large-area applications. The successful demonstration of M3D inverters based on large-scale 2D monolayer TMDs indicate their high potential for advancing the application of 2D TMDs in future integrated circuits.

Ultrafast and Broad-Band Graphene Heterojunction Photodetectors with High Gain.

Graphene possesses an exotic band structure that spans a wide range of important technological wavelength regimes for photodetection, all within a single material. Conventional methods aimed at enhancing detection efficiency often suffer from an extended response time when the light is switched off. The task of achieving ultrafast broad-band photodetection with a high gain remains challenging. Here, we propose a devised architecture that combines graphene with a photosensitizer composed of an alternating strip superstructure of WS2-WSe2. Upon illumination, n+-WS2 and p+-WSe2 strips create alternating electron- and hole-conduction channels in graphene, effectively overcoming the tradeoff between the responsivity and switch time. This configuration allows for achieving a responsivity of 1.7 × 107 mA/W, with an extrinsic response time of 3-4 µs. The inclusion of the superstructure booster enables photodetection across a wide range from the near-ultraviolet to mid-infrared regime and offers a distinctive photogating route for high responsivity and fast temporal response in the pursuit of broad-band detection.

Silicon-van der Waals heterointegration for CMOS-compatible logic-in-memory design.

Silicon CMOS-based computing-in-memory encounters design and power challenges, especially in logic-in-memory scenarios requiring nonvolatility and reconfigurability. Here, we report a universal design for nonvolatile reconfigurable devices featuring a 2D/3D heterointegrated configuration. By leveraging the photo-controlled charge trapping/detrapping process and the partially top-gated energy band landscape, the van der Waals heterostacking achieves polarity storage and logic reconfigurable characteristics, respectively. Precise polarity tunability, logic nonvolatility, robustness against high temperature (at 85°C), and near-ideal subthreshold swing (80 mV dec-1) can be done. A comprehensive investigation of dynamic charge fluctuations provides a holistic understanding of the origins of nonvolatile reconfigurability (a trap level of 1013 cm-2 eV-1). Furthermore, we cascade such nonvolatile reconfigurable units into a monolithic circuit layer to demonstrate logic-in-memory computing possibilities, such as high-gain (65 at Vdd = 0.5 V) logic gates. This work provides an innovative 3D heterointegration prototype for future computing-in-memory hardware.

Highly hydroxylated hafnium clusters are accessible to high resolution EUV photoresists under small energy doses.

This work reports the success in accessing high-resolution negative-tone EUV photoresists without radical chain growth in the aggregation mechanism. The synthesis of a highly hydroxylated Hf6O4(OH)8(RCO2)8 cluster 3 (R = s-butyl or s-Bu) is described; its EUV performance enables high resolution patterns HP = 18 nm under only 30 mJ cm-2. This photoresist also achieves high resolution patterns for e-beam lithography. Our new photoresist design to increase hydroxide substitutions of carboxylate ligands in the Hf6O4(OH)4(RCO2)12 clusters improves the EUV resolution and also greatly reduces EUV doses. Mechanistic analysis indicates that EUV light not only enables photolytic decomposition of carboxylate ligands, but also enhances the Hf-OH dehydration. One additional advantage of cluster 3 is a very small loss of film thickness (ca. 13%) after the EUV pattern development.

Molybdenum Chloride Nanostructures with Giant Lattice Distortions Intercalated into Bilayer Graphene.

The nanospace of the van der Waals (vdW) gap between structural units of two-dimensional (2D) materials serves as a platform for growing unusual 2D systems through intercalation and studying their properties. Various kinds of metal chlorides have previously been intercalated for tuning the properties of host layered materials, but the atomic structure of the intercalants remains still unidentified. In this study, we investigate the atomic structural transformation of molybdenum(V) chloride (MoCl5) after intercalation into bilayer graphene (BLG). Using scanning transmission electron microscopy, we found that the intercalated material represents MoCl3 networks, MoCl2 chains, and Mo5Cl10 rings. Giant lattice distortions and frequent structural transitions occur in the 2D MoClx that have never been observed in metal chloride systems. The trend of symmetric to nonsymmetric structural transformations can cause additional charge transfer from BLG to the intercalated MoClx, as suggested by our density functional theory calculations. Our study deepens the understanding of the behavior of matter in the confined space of the vdW gap in BLG and provides hints at a more efficient tuning of material properties by intercalation for potential applications, including transparent conductive films, optoelectronics, and energy storage.

Novel hexameric tin carboxylate clusters as efficient negative-tone EUV photoresists: high resolution with well-defined patterns under low energy doses.

Synthesis of two novel tin carboxylate clusters (RSn)6(R'CO2)8O4Cl2 is described, and their structures have been characterized by X-ray diffraction. These clusters have irregular ladder geometry to form very smooth films with small surface roughness (RMS <0.7 nm) over a large domain. EUV lithography can be used to resolve half pitches (HPs) in the order of 15-16 nm with line width roughness (LWR = 4.5-6.0 nm) using small doses (20-90 mJ cm-2). Cluster 1 (R = n-butyl; R'CO2 = 2-methyl-3-butenoate) contains only a radical precursor and cluster 2 (R = vinyl, R'CO2 = 2-methylbutyrate) bears both a radical precursor and an acceptor; the latter is much better than the former in EUV and e-beam photosensitivity. For these clusters, the mechanisms of EUV irradiation have been elucidated with high resolution X-ray photoelectron spectroscopy (HRXPS) and reflective Fourier-transform infrared spectroscopy (FTIR). At low EUV doses, two clusters undergo a Sn-Cl bond cleavage together with a typical decarboxylation to generate carbon radicals. The n-butyl groups of cluster 1 are prone to cleavage whereas the vinyl-Sn bonds of species 2 are inert toward EUV irradiation; participation of radical polymerization is evident for the latter.

Patterning and doping of transition metals in tungsten dichalcogenides.

Substitutional transition metal doping in two-dimensional (2D) layered dichalcogenides is of fundamental importance in manipulating their electrical, excitonic, magnetic, and catalytic properties through the variation of the d-electron population. Yet, most doping strategies are spatially global, with dopants embedded concurrently during the synthesis. Here, we report an area-selective doping scheme for W-based dichalcogenide single layers, in which pre-patterned graphene is used as a reaction mask in the high-temperature substitution of the W sublattice. The chemical inertness of the thin graphene layer can effectively differentiate the spatial doping reaction, allowing for local manipulation of the host 2D materials. Using graphene as a mask is also beneficial in the sense that it also acts as an insertion layer between the contact metal and the doped channel, capable of depinning the Fermi level for low contact resistivity. Tracing doping by means of chalcogen labelling, deliberate Cr embedment is found to become energetically favorable in the presence of chalcogen deficiency, assisting the substitution of the W sublattice in the devised chemical vapor doping scheme. Atomic characterization using scanning transmission electron microscopy (STEM) shows that the dopant concentration is controllable and varies linearly with the reaction time in the current doping approach. Using the same method, other transition metal atoms such as Mo, V, and Fe can also be doped in the patterned area.

Bifunctional Monolayer WSe2/Graphene Self-Stitching Heterojunction Microreactors for Efficient Overall Water Splitting in Neutral Medium.

Developing efficient bifunctional electrocatalysts in neutral media to avoid the deterioration of electrodes or catalysts under harsh environments has become the ultimate goal in electrochemical water splitting. This work demonstrates the fabrication of an on-chip bifunctional two-dimensional (2D) monolayer (ML) WSe2/graphene heterojunction microreactor for efficient overall water splitting in a neutral medium (pH = 7). Through the synergistic atomic growth of the metallic Cr dopant and graphene stitching contact on the 2D ML WSe2, the bifunctional WSe2/graphene heterojunction microreactor consisting of a full-cell configuration demonstrates excellent performance for overall water splitting in a neutral medium. Atomic doping of metallic Cr atoms onto the 2D ML WSe2 effectively facilitates the charge transfer at the solid-liquid interface. In addition, the direct growth of the self-stitching graphene contact with the 2D WSe2 catalyst largely reduces the contact resistance of the microreactor and further improves the overall water splitting efficiency. A significant reduction of the overpotential of nearly 1000 mV at 10 mA cm-2 at the Cr-doped WSe2/graphene heterojunction microreactor compared to the ML pristine WSe2 counterpart is achieved. The bifunctional WSe2/graphene self-stitching heterojunction microreactor is an ideal platform to investigate the fundamental mechanism of emerging bifunctional 2D catalysts for overall water splitting in a neutral medium.

Diffused Beam Energy to Dope van der Waals Electronics and Boost Their Contact Barrier Lowering.

Contact engineering of two-dimensional semiconductors is a central issue for performance improvement of micro-/nanodevices based on these materials. Unfortunately, the various methods proposed to improve the Schottky barrier height normally require the use of high temperatures, chemical dopants, or complex processes. This work demonstrates that diffused electron beam energy (DEBE) treatment can simultaneously reduce the Schottky barrier height and enable the direct writing of electrical circuitry on van der Waals semiconductors. The electron beam energy projected into the region outside the electrode diffuses into the main channel, producing selective-area n-type doping in a layered MoTe2 (or MoS2) field-effect transistor. As a result, the Schottky barrier height at the interface between the electrode and the DEBE-treated MoTe2 channel is as low as 12 meV. Additionally, because selective-area doping is possible, DEBE can allow the formation of both n- and p-type doped channels within the same atomic plane, which enables the creation of a nonvolatile and homogeneous MoTe2 p-n rectifier with an ideality factor of 1.1 and a rectification ratio of 1.3 × 103. These results indicate that the DEBE method is a simple, efficient, mask-free, and chemical dopant-free approach to selective-area doping for the development of van der Waals electronics with excellent device performances.

Reversible Charge-Polarity Control for Multioperation-Mode Transistors Based on van der Waals Heterostructures.

Van der Waals (vdW) heterostructures-in which layered materials are purposely selected to assemble with each other-allow unusual properties and different phenomena to be combined and multifunctional electronics to be created, opening a new chapter for the spread of internet-of-things applications. Here, an O2 -ultrasensitive MoTe2 material and an O2 -insensitive SnS2 material are integrated to form a vdW heterostructure, allowing the realization of charge-polarity control for multioperation-mode transistors through a simple and effective rapid thermal annealing strategy under dry-air and vacuum conditions. The charge-polarity control (i.e., doping and de-doping processes), which arises owing to the interaction between O2 adsorption/desorption and tellurium defects at the MoTe2 surface, means that the MoTe2 /SnS2 heterostructure transistors can reversibly change between unipolar, ambipolar, and anti-ambipolar transfer characteristics. Based on the dynamic control of the charge-polarity properties, an inverter, output polarity controllable amplifier, p-n diode, and ternary-state logics (NMIN and NMAX gates) are demonstrated, which inspire the development of reversibly multifunctional devices and indicates the potential of 2D materials.

Electric control of valley polarization in monolayer WSe2 using a van der Waals magnet.

Electrical manipulation of the valley degree of freedom in transition metal dichalcogenides is central to developing valleytronics. Towards this end, ferromagnetic contacts, such as Ga(Mn)As and permalloy, have been exploited to inject spin-polarized carriers into transition metal dichalcogenides to realize valley-dependent polarization. However, these materials require either a high external magnetic field or complicated epitaxial growth steps, limiting their practical applications. Here we report van der Waals heterostructures based on a monolayer WSe2 and an Fe3GeTe2/hexagonal boron nitride ferromagnetic tunnelling contact that under a bias voltage can effectively inject spin-polarized holes into WSe2, leading to a population imbalance between ±K valleys, as confirmed by density functional theory calculations and helicity-dependent electroluminescence measurements. Under an external magnetic field, we observe that the helicity of electroluminescence flips its sign and exhibits a hysteresis loop in agreement with the magnetic hysteresis loop obtained from reflective magnetic circular dichroism characterizations on Fe3GeTe2. Our results could address key challenges of valleytronics and prove promising for van der Waals magnets for magneto-optoelectronics applications.

WSe2/WS2 Heterobilayer Nonvolatile Memory Device with Boosted Charge Retention.

A two-dimensional (2D) nonvolatile memory device architecture to improve the long-term charge retention with the minimum charge loss without compromising storage capacity and the extinction ratio for practical applications has been an imminent demand. To address the current issue, we adopted a novel type-II band-aligned heterobilayer channel comprising vertically stacked monolayer WSe2 nanodots on monolayer WS2. The band offset modulation leads to electron doping from WSe2 nanodots into the WS2 channel without any external driving electric field. As a result, the tested device outperformed with a memory window as high as 34 V and a negligible charge loss of 7% in a retention period of 10 years while maintaining a high extinction ratio of 106. The doping technique presented in this work provides a feasible route to modulate the electrical properties of 2D channel materials without hampering charge transport, paving the way for high-performance 2D memory devices.

Oxidation and Degradation of WS2 Monolayers Grown by NaCl-Assisted Chemical Vapor Deposition: Mechanism and Prevention.

The preservation of two-dimensional WS2 in the environment is a concern for researchers. In addition to water vapor and oxygen, the latest research points out that degradation is directly related to light absorption. Based on the selection rules of nonlinear optics, two-photon absorption is dipole forbidden in the exciton 1s states, but second-harmonic generation (SHG) is allowed with virtual transitions. According to this mechanism, we proved that SHG is an optical detection method with non-photooxidative damage and energy characteristics. With this detection method, we can explore the oxidation and degradation mechanisms of WS2 grown by NaCl-assisted chemical vapor deposition in its original state. The WS2 monolayers that use NaCl to assist in growth have undergone different degradation processes, starting to oxidize from random positions in the triangular flake. We use a photocatalytic reaction to explain the photo-induced degradation mechanism with sulfur vacancies. It was further found that WS2 grown with NaCl assistance is hydrolyzed in a dark and high-humidity environment, which does not occur in pure WS2. Finally, we demonstrated that changing the direction of the sapphire substrate relative to the gas flow direction to grow NaCl-assisted WS2 can greatly improve its stability in the ambient atmosphere, even when exposed to light. The optimal geometric structures and ground state energies are investigated by the density functional theory-based calculations. According to the orientation and symmetry of NaCl-assisted WS2, we can expect that it will have a better growth quality when the gas flow direction is perpendicular to the [112̄0] direction of the sapphire substrate. This contributes to the nucleation and subsequent growth of NaCl-assisted WS2. This research provides a more stable optical inspection method than other established methods and greatly improves the operational stability of NaCl-assisted WS2 under environmental conditions.

Resonance Raman enhancement by the intralayer and interlayer electron-phonon processes in twisted bilayer graphene.

Twisted bilayer graphene is a fascinating system due to the possibility of tuning the electronic and optical properties by controlling the twisting angle [Formula: see text] between the layers. The coupling between the Dirac cones of the two graphene layers gives rise to van Hove singularities (vHs) in the density of electronic states, whose energies vary with [Formula: see text]. Raman spectroscopy is a fundamental tool to study twisted bilayer graphene (TBG) systems since the Raman response is hugely enhanced when the photons are in resonance with transition between vHs and new peaks appear in the Raman spectra due to phonons within the interior of the Brillouin zone of graphene that are activated by the Moiré superlattice. It was recently shown that these new peaks can be activated by the intralayer and the interlayer electron-phonon processes. In this work we study how each one of these processes enhances the intensities of the peaks coming from the acoustic and optical phonon branches of graphene. Resonance Raman measurements, performed in many different TBG samples with [Formula: see text] between [Formula: see text] and [Formula: see text] and using several different laser excitation energies in the near-infrared (NIR) and visible ranges (1.39-2.71 eV), reveal the distinct enhancement of the different phonons of graphene by the intralayer and interlayer processes. Experimental results are nicely explained by theoretical calculations of the double-resonance Raman intensity in graphene by imposing the momentum conservation rules for the intralayer and the interlayer electron-phonon resonant conditions in TBGs. Our results show that the resonant enhancement of the Raman response in all cases is affected by the quantum interference effect and the symmetry requirements of the double resonance Raman process in graphene.

Embedment of Multiple Transition Metal Impurities into WS2 Monolayer for Bandstructure Modulation.

Band structure by design in 2D layered semiconductors is highly desirable, with the goal to acquire the electronic properties of interest through the engineering of chemical composition, structure, defect, stacking, or doping. For atomically thin transition metal dichalcogenides, substitutional doping with more than one single type of transition metals is the task for which no feasible approach is proposed. Here, the growth of WS2 monolayer is shown codoped with multiple kinds of transition metal impurities via chemical vapor deposition controlled in a diffusion-limited mode. Multielement embedment of Cr, Fe, Nb, and Mo into the host lattice is exemplified. Abundant impurity states thus generate in the bandgap of the resultant WS2 and provide a robust switch of charging/discharging states upon sweep of an electric filed. A profound memory window exists in the transfer curves of doped WS2 field-effect transistors, forming the basis of binary states for robust nonvolatile memory. The doping technique presented in this work brings one step closer to the rational design of 2D semiconductors with desired electronic properties.

Formation of Highly Doped Nanostripes in 2D Transition Metal Dichalcogenides via a Dislocation Climb Mechanism.

Doping of materials beyond the dopant solubility limit remains a challenge, especially when spatially nonuniform doping is required. In 2D materials with a high surface-to-volume ratio, such as transition metal dichalcogenides, various post-synthesis approaches to doping have been demonstrated, but full control over spatial distribution of dopants remains a challenge. A post-growth doping of single layers of WSe2 is performed by adding transition metal (TM) atoms in a two-step process, which includes annealing followed by deposition of dopants together with Se or S. The Ti, V, Cr, and Fe impurities at W sites are identified by using transmission electron microscopy and electron energy loss spectroscopy. Remarkably, an extremely high density (6.4-15%) of various types of impurity atoms is achieved. The dopants are revealed to be largely confined within nanostripes embedded in the otherwise pristine WSe2 . Density functional theory calculations show that the dislocations assist the incorporation of the dopant during their climb and give rise to stripes of TM dopant atoms. This work demonstrates a possible spatially controllable doping strategy to achieve the desired local electronic, magnetic, and optical properties in 2D materials.

Design of Core-Shell Quantum Dots-3D WS2 Nanowall Hybrid Nanostructures with High-Performance Bifunctional Sensing Applications.

Transition metal dichalcogenides (TMDCs) have recently attracted a tremendous amount of attention owing to their superior optical and electrical properties as well as the interesting and various nanostructures that are created by different synthesis processes. However, the atomic thickness of TMDCs limits the light absorption and results in the weak performance of optoelectronic devices, such as photodetectors. Here, we demonstrate the approach to increase the surface area of TMDCs by a one-step synthesis process of TMDC nanowalls from WOx into three-dimensional (3D) WS2 nanowalls. By utilizing a rapid heating and rapid cooling process, the formation of 3D nanowalls with a height of approximately 150 nm standing perpendicularly on top of the substrate can be achieved. The combination of core-shell colloidal quantum dots (QDs) with three different emission wavelengths and 3D WS2 nanowalls further improves the performance of WS2-based photodetector devices, including a photocurrent enhancement of 320-470% and shorter response time. The significant results of the core-shell QD-WS2 hybrid devices can be contributed by the high nonradiative energy transfer efficiency between core-shell QDs and the nanostructured material, which is caused by the spectral overlap between the emission of core-shell QDs and the absorption of WS2. Besides, outstanding NO2 gas-sensing performance of core-shell QDs/WS2 devices can be achieved with an extremely low detection limit of 50 ppb and a fast response time of 26.8 s because of local p-n junctions generated by p-type 3D WS2 nanowalls and n-type core-shell CdSe-ZnS QDs. Our work successfully reveals the energy transfer phenomenon in core-shell QD-WS2 hybrid devices and shows great potential in commercial multifunctional sensing applications.